The present invention relates to long point self-drilling screws and the method of making such screws.
Long point drill screws or self-drilling screws have many advantages in applicable situations. Self-drilling screws eliminating the need to drill a hole prior to driving a screw thereby eliminating the labor and tooling inherent in a drill before driving operation. While the additional step of drilling is eliminated by such a screw, the hole drilled by the self-drilling screw must be highly accurate to prevent undesirable results.
For example, if the drilled hole is too large, there will not be sufficient material for proper thread engagement. Depending on the degree of enlargement of the hole, it is possible that the screw may strip out of the workpiece. Clearly, it is undesirable to have screws stripping out of workpieces. Further, if the drilled hole is too small, the threads are forced to form or cut through too much material and may result in torsional failure or "snapping-off" of the screw. If the screw snaps-off in the drilled hole, the shank of the screw must be drilled out and a new screw inserted therein. Correction of a snapped-off screw is very time consuming and may not be possible depending on the circumstances.
Further, it is desirable to form a self-drilling screw through a cold-working process as opposed to a cutting process. A screw formed through a cutting process will not maximize the benefits of the grain structure inherent in the screw blank metal. In contrast, a cold-formed screw maintains the grain structure and cold works the material thereby maximizing the structural benefits of the material. Additionally, cold-forming the screw eliminates the additional machining processes, cutting tooling and scrap material of a typical cutting process.
An example of a method and apparatus for producing drill screws is shown in U.S. Pat. No. 4,241,639 to Baer. The method and apparatus as shown in Baer produces a self-drilling screw by a method employing numerous cutting operations. As shown in Baer, the cutting operations are relatively complex and requires numerous pieces of precision positioning and cutting equipment. Further, the product resulting from the process as shown in Baer is a screw in which the inherent material strength is not maximized as a result of the cutting process.
A common problem with self-drilling screws, especially when the drilling portion of the screw is rather long, is that the screw tends to drill an oversized hole. An oversized hole is formed by a long point self-drilling screw when the drilling portion is not substantially straight. A drilling portion which is not substantially straight will tend to wobble while the screw is driven producing an elliptical or bell-mouthed hole or both. Such an irregular hole typically will not meet required tolerances for satisfactory thread forming and engagement thereby increasing the possibility of the screw stripping-out of the workpiece.
In an attempt to control irregularly shaped holes, screw manufacturers formed the drill portion with a working edge as previously formed and a trailing edge of the drilling portion which was slightly relieved from the circumference of the shank. An example of such a prior art self-drilling screw is provided in FIGS. 8 and 9. As illustrated in FIGS. 8 and 9, a flute is formed in the shank of a screw with a working edge and a trailing edge. A working edge which is not substantially straight tends to gouge the hole unless the trailing edge is relieved from the circumference of the shank (see FIG. 9) to compensate for the irregularity of the shank.
The resulting drilling portion of the screw is not round since the relieved trailing edge has a diameter which is slightly smaller than the working edge. As a result, this type of screw tends to be unstable and wobble when driven because only the two working edges contact the material while the screw is driven. Therefore, the apparent solution to the problem created by an irregular drilling portion created another problem producing essentially the same undesirable results.